Pseudomonas aeruginosa is a ubiquitous and important opportunistic pathogen that possesses an impressive arsenal of virulence factors. Two quorum-sensing systems, las and rhl, have been identified as controlling production of several of these virulence factors and has become a model for studying cell-density control of virulence genes. Evidence that the quorum-sensing systems of P. aeruginosa play a major role in twitching motilitiy is presented here for the first time. Recently, P. aeruginosa has also become the preferred model system for studies of type-4 pili and twitching motility. The polar, type-4 pili of P. aeruginosa are multifunctional structures which play a crucial role in the virulence of this organism by promoting adherence to, and colonization of, a variety of tissues as well as being the organelles responsible for twitching motility. The overall goal of this research is to work out the cellular and molecular basis for the role of quorum sensing in the flagella-independent, pili-associated mode of bacterial surface translocation known as 'twitching motility' in P. aeruginosa. A primary goal of this project is to identify the twitching motility gene(s) that are controlled by the quorum-sensing systems of P. aeruginosa. The cellular and molecular mechanisms of twitching motility are not well understood, nor is it clear how the quorum-sensing systems affect these mechanisms. The mechanisms underlying twitching motility, and its connection to quorum-sensing will be addressed at the genetic, biochemical and cell-cell interaction levels. Finally, the contributions quorum-sensing and twitching motility make to the frequency of conjugal mating in P. aeruginosa on semi-solid surfaces will be examined for the first time. Standard plate matings between wild-type cells and the various twitching motility and quorum-sensing mutants will be conducted. In addition, the temporal and spatial nature of gene transfer among members of the populations on semi-solid surfaces will be studies at the single cell level using synthesis of GFP to follow conjugal transfer.